1. Field of the Invention
This invention is directed toward intervertebral body fusion devices, more particularly to expandable intervertebral body fusion devices.
2. Related Art
Presently, most implanted fusion cages are non-expandable devices although many expandable fusion cage designs are found in the patent literature. The current art discloses jacks, dilators, threaded expanders, cams, pistons and/or packable/inflatable expansion means.
Several challenges exist with currently marketed fixed-form fusion cages if adequate care is not taken during insertion. One challenge is damage to the cage or vertebral body upon insertion into and/or distraction of the intervertebral disc space. Another challenge exists for potential spinal fusion device expulsion when large annular defects are created for insertion of these devices. Yet another challenge is present when over-distraction of vertebral bodies is required to insert fixed-form cages which causes reduction of annular tension as well as soft tissue balance. More specifically, over-distraction is when the intervertebral disc space height is expanded in the axial plane beyond the normal disc space height, stretching the annulus as well as adjacent soft tissues. This results when a non-expandable/fixed form lordotic cage(s) are inserted into the disc space from the posterior or posterior lateral direction. The lordotic angle on the cage creates an anterior disc space height that is larger then the posterior disc space height. The disc space is expanded (or over distracted) to the larger anterior height in order to insert a intervertebral device into the disc space. This overdistraction reduces compressive forces on the cage following insertion. Therefore, it would be advantageous to provide an intervertebral disc cage that does not cause overdistraction of the disc space.
A fair portion of the art in expandable interbody fusion devices are related to cylindrical or conical devices.
US 2005/0021041 and US 2002/0143401 relate to interbody fusion implants that are radially expandable at one of the leading or trailing ends to expand both the height and the width of the implant. US 2002/0107574 is directed toward an expandable intervertebral having a rod for insertion into a tapered bore of an implant to expand the distance between the top portion and bottom portion of the implant. US 2001/0034553 is related to a push-in interbody spinal fusion implant having expandable height and having a frusto-conical shape or shape of cylinder split along a horizontal plane through its mid-longitudinal axis with an upper member and a lower member angled to each other during insertion into the spine. U.S. Pat. No. 7,018,415 is directed to an expandable intervertebral disc space implant having an expansion member such as a cam or rack and pinion.
This invention differs from prior art because it employs a non-disclosed expansion means to increase disc height. This means is the rotation of the inner cylindrical or conical axially nested endplate within the outer endplate. Historical art utilizes jacks, dilators, threaded expanders, cams, pistons and/or packable/inflatable expansion means.